Self-lubricating compositions



March 8, 1966 M. E. CAMPBELL ET AL 3,239,288

SELF-LUBRICATING COMPOSITIONS Filed Oct. 24, 1963 INVENTORS JAN M VAN WYK ATTORNEY United States Patent 3,239,288 SELF-LUBRICATING COMPOSITIONS Mahlon E. Campbell, Bellevue, and Jan W. Van Wyk, Kirkland, Wash., assignors to The Boeing Company,

Seattle, Wash., a corporation of Delaware Filed Oct. 24, 1963, Ser. No. 318,565 16 Claims. (Cl. 308-199) This invention relates to self contained, high load capacity self-lubricating compositions suitable for use over a temperature range of 300 F. to 800 F. in air and -300 F. to 1500 F. in vacuum.

Current technology developments have put great emphasis on systems having operating capacity at high temperatures and at cryogenic temperatures with some few systems having both requirements. The high temperature field has received much study and emphasis from repeated space activities. The expanding temperature ranges of operating systems along with confined space and desired low weight have caused additional problems in normal functioning systems. These major problems, along with related difficulties, have made it necessary to develop compositions which possess self-lubricating properties due to their innate dry film lubricating properties.

Therefore it is an object of this invention to fabricate a dependable self-lubricating material.

It is another object of this invention to provide a material which will have self-lubricating properties at cryogenic temperatures and at elevated temperatures up to 800 F. in air and 1500" F. in vacuum.

It is a still further object of this invention to provide a material having lubricating properties in air and in vacuum.

Still further objects and applications of this invention will become apparent from the following description and appended claims.

FIGURE 1 represents a bearing which was used to test the lubrication properties of our compositions.

FIGURE 2 represents the parts of the bearing in FIG- URE 1 when assembled.

The self-lubricating film on materials has been the object of much research with many resulting developments. Some of these developments include recognizing the following substances as possessing self-lubricating films; the sulfides and tellurides of molybdenum and tungsten, lead oxide and titanium dioxide. Techniques for using these materials include impregnating porous structures with these materials or fabricating solid materials out of these constituents. In addition to these developments, several organic chemical materials have been used to supply a lubrication phenomenon when incorporated into the matrix of a material.

From the above survey of the existing art of selflubricating properties of materials, it is easy to see that most of the materials lack high temperature stability, mechanical strength, forming properties or lose their lubricity at high temperatures or cryogenic temperatures.

We have developed the following ranges of compositions to overcome these deficiencies of the prior art.

COMPOSITION 1 Constituent: Percentage by weight Molybdenum disulfide (M08 70 to 90 Iron (Fe) to 27 Platinum (Pt) 1to 3 COMPOSITION 2 Constituent: Percentage by weight Molybdenum disulfide (MoS 70 to 90 Iron (Fe) to 25 Palladium (Pd) 2 to 5 COMPOSITION 3 Nickel (Ni) 15 to 45 The self-lubricating properties of molybdenum disulfide when alone or alloyed with other materials are known. The lubricating properties of molybdenum disulfide exhibit a wide temperature range from 300' to 800 F. in air and 300 to 1500 F. in vacuum. The crystals of molybdenum disulfide exhibit a plate structure in which successive plates of molybdenum atoms are arranged with two successive layers of sulfur atoms between each layer of molybdenum atoms. The atoms in each layer lie in a plane hexagonal array. A large crystal of molybdenum disulfide is built up of layers of molybdenum attached by strong ionic linkages to adjacent layers of sulfur while the adjacent sulfur layers are held together only by weak homopolar linkage bonds. Thus the crystal consists of laminae having a central molybdenum atom array strongly bound to sulfur atoms on each side. These laminae of molybdenum disulfide therefore may 'be easily separated by mechanical forces into flat plates that have a flaky appearance and have a greasy feel.

While the sulfur layers have only a weak attraction for each other in forming a complete crystal of molybdenum disulfide, the sulfur atoms have a much greater afiinity for metals. Therefore, the molybdenum disulfide plates will attach themselves under certain conditions quite firmly to metals. The sulfur atoms, having weak affinity for each other, will not be held by as great a force as those holding the sulfur to the metal, or even the metal to metal adhesive forces. Therefore the sulfide present serves as a lubricating mechanism.

It is scientifically known fact that molybdenum, when in a compound, exhibits very stable tendencies at high and low temperatures. Molybdenum disulfide is no exception to this scientific principle of stability of molybdenum compounds with the additional advantage of the lubricity of the sulfide layers of the compound. However, the molybdenum disulfide lacks many of the metallic properties essential .to mechanical applications among which are ductility, compressive strength, tensile strength, malleability, shock resistance, and absence of a brittle failure. Instead one finds molybdenum disulfide exhibiting the non-metallic properties characteristic of the slag group which properties are the opposite of the desired metallic properties. This leaves the problem of finding a suitable matrix for suspending the molybdenum disulfide therein. This matrix must lend the above mechanical properties to the composition plus allowing the molybdenum disulfide to impart its lubricity and high temperature stability to the matrix. These objectives have been achieved by the above compositions employing molybdenum disulfide. The resulting compositions are so phenomenal that one can use them in assemblies from 300 to 800 F. in air or 300 to 1500 F. in vacuum with reliance on their self-lubricating properties.

Iron is used in the first three compositions in which molybdenum disulfide is a lubricant. This material has a body centered cubic crystal structure with the result that the iron in the matrix of these compositions acts as a binder. The iron imparts several of its properties including malleability, ductility, and shock resistance to the properties of the matrix. However, the most important function of the iron is to impart strength to the matrix. The iron serves as a binder in helping to hold the self-lubricating compositions together. In this manner the compositions utilizing iron as a binder have the advantage of a ferrous base and the strength properties and related properties that are associated with iron. Iron is also used because its sulfide is very unstable; thus iron does not tend to reduce the molybdenum disulfide.

Platinum, palladium and molybdenum present in small quantities in the first three compositions serve primarily to supplement the action of iron as a binder and provide higher strength materials at elevated temperatures plus the function of making the iron and molybdenum disulfide physically compatible in the same material. These three binders are more or less equivalents except that platinum seems to be a more efficient binder than paladium and the palladium seems to be more efficient as a binder than the molybdenum. This merely means that a smaller quantity of the more efficient binder will serve the same function as a larger amount of a less efficient binder. It is well to consider that a less efficient binder may have other properties, such as ductility, etc., which may offset this disadvantage of being a less efiicient binder.

As used in compositions 4 and 5, titanium dioxide exhibits a physical structure which differs from the physical structure of molybdenum disulfide, but manifests self-lubricating properties very similar to molybdenum disulfides self-lubricating properties. It is difficult to account for the similar self-lubricating properties of these two different materials because of the very different crystal structure. Titanium dioxide is known to exist in several different crystallographic forms. However, much less is known about the lubrication. mechanism of titanium dioxide than of the molybdenum disulfide. It has been found that the rutile form of titanium dioxide exhibits the lubrication mechanism required in this invention. Titanium dioxide of a rutile-type structure forms irreversibly upon heating titanium dioxide to the temperature of 915 C. This crystal structure of the rutile-type titanium dioxide is not a simple layer system as molybdenum disulfide and therefore the mechanisms of lubrication cannot be similar. One explanation attempts to account for the lubrication mechanism by suggesting that its low friction characteristics may be due to slip on its dislocation slip system. However, questions of low dislocation mobility and of intercrystalline orientation would then require explanation. The manifestation of a piezoelectric effect of the rutile form of titanium dioxide is another possible explanation of the lubrication mechanism. A third possibility of explanation of the lubrication mechanism is the type of crystalline surface for-med by the reaction of the unsatisfied bonds of the titanium with the surrounding atmosphere. While the explanation of the lubrication mechanism of the rutile-type of titanium dioxide is unknown, the property is adequately utilized in the practice of this invention.

It is a scientifically known fact that titanium, when in a compound form such as titanium dioxide, exhibits very stable tendencies at high temperatures and low temperatures. In addition to its stability, titanium dioxide exhibits lubricity over a temperature range from 900 F. to 1500 F. However, titanium dioxide lacks many of the metallic properties essential to mechanical applications among which are ductility, compressive strength, tensile strength, malleability, shock resistance and absence of a brittle failure. Instead one finds titanium dioxide exhibiting the non-metallic properties characteristie of the ceramic group which properties are almost always the opposite of the desired metallic properties. This leaves the problem of finding a suitable matrix for suspending the titanium dioxide therein. This matrix must lend the above mechanical properties to the composition plus allowing the titanium dioxide to impart its lubricity and high temperature stability to the matrix. These objectives have been achieved by the above compositions employing titanium dioxide. It should also be noted that the self-lubrication properties of these materials extend throughout the matrix of the compositions.

In compositions 4 and 5, the titanium dioxide serves as the primary solid lubricant but nickel oxide and molybdenum disulfide also serve as lubricants in these compositions. Titanium dioxide, as noted above, is not as efficient a lubricant as molybdenum disulfide but it has the advantage of being easier to sinter and it withstands higher temperatures than a molybdenum disulfide base material.

The nickel metal present in compositions 4 and 5 is there for the primary purpose of serving as a binder and holding the compositions together. Nickel also has several properties similar to iron so that the compositions have mechanical properties in addition to their self-lubricating properties due to the presence of nickel. Nickel has proven very compatible for use in compositions where titanium dioxide is present. Nickel does not have the aflinity for oxygen that titanium does so the nickel remains in the composition as an element and does not reduce the titanium dioxide. Even if a nickel oxide were formed this would be helpful because of its high temperature lubrication properties as noted below.

Nickel oxide in compositions 4 and 5 serves a dual purpose in that it acts as a binder and at higher temperatures (1000 to 1500 F.) it develops great lubrication properties which supplement the lubrication properties of titanium dioxide. Nickel oxide has some properties very similar to nickel in that the properties of nickel oxide are more closely related to the mechancal advantages of a metal than to the brittle nature of a ceramic. This explains the' binder characteristics exhibited by nickel oxide in compositions 4 and 5.

Molybdenum disulfide in composition 5 serves the same function as described previously in relation to the first three compositions and the discussion will not be repeated.

Lubrication problems in a vacuum may seem no different than in air, but actually vacuum conditions impose more serious problems and more rapid wear on any assembly than operation in air does. If a normal lubricant is introduced onto the surfaces of material in assemblies operating in vacuum conditions, this lubricant will evaporate due to its high vapor pressure. Thus in short periods of time the lubricant would be non-existent.

Any assembly, formed of metal and run in a vacuum, behaves differently than the same assembly operated in air. It has been found that the absence of an oxide film on the surfaces of the. assembly member causes cohesion of metal between the several parts of the assembly while operating in the vacuum. The metal. will adhere or cohere with other metallic surfaces as cohesive contact is made and broken with the other metallic surfaces during operation. The transfer of metal particles occurs rapidly and visible pitting will be evident in a short time. This results in the metal surfaces becoming rough, increasing the noise level and increasing needs in operating power. In air this cohesion which effects the roughening of a hearing in a vacuum does not take place due to the presence of an oxide film on the surfaces of the metal. The use of a self-lubricating material eliminates the need for an oxide surface -film for lubrication in a vacuum as well as providing a dry film lubricant in air.

The compositions of this invention are fabricated by a hot pressing operation which provides the required high strength. To insure the development of the ideal properties of these compositions we practice the following steps in hot pressing. After the components are properly mixed together the composite is heated to 350 F. thus volatilizing any water present. After heating to 350 F., the composite is again mixed followed by pressing the components at room temperatures into graphite dies. The graphite dies are constructed to give the composition its desired final configuration. Next the dies are preheated to approximately 1200 F. followed by hot pressing the composition anywhere in the temperature range of 1600 to 2500 F. Following the hot pressing step, we cool under load to 1400 F.; then we remove the pressure from the dies and cool from 1400 to 500 F. and remove the composition from the graphite die. We practice temperature and time variations in this process as we change the relative proportions in our composition.

Example 1 The following material, typical of composition 1, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Molybdenum disulfide (MoS 90' Iron (Fe) 8 Platinum (Pt) 2 Example 2 The following material, typical of composition 2, was formulated by the hot pressing technique described above.

Constituent:

Percentage by weight Molybdenum disulfide (M08 80 Iron (Fe) 16 Palladium (Pd) 4 Example 3 The following material, typical of composition 3, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight 1 Molybdenum disulfide (M08 75 Iron (Fe) 16.25

.Molybdenum (Mo) 8.75

i Example 4 The following material, typical of composition 4, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Titanium dioxide (TiO 45 Nickel oxide (NiO) l.

Nickel (Ni) 40 Example 5 The following material, typical of composition 5, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight The following material, typical of composition 1, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Molybdenum disulfide (M05 80 Iron (Fe) 19 Platinum (Pt) 1 Example 7 The following material, typical of composition 2, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Molybdenum disulfide (MoS 88 Iron (Fe) 10 Palladium (Pd) 2 Example 8 The following material, typical of composition 3, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Molybdenum disulfide (M08 70 Iron (Fe) 2O Molybdenum (Mo) 10 Example 9 The following material, typical of composition 4, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Titanium dioxide (TiO 47.5

Nickel oxide (NiO) 10 Nickel (Ni) 42.5

Example 10 The following material, typical of composition 5, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight The following material, typical of composition 1, was formulated by hot pressing technique described above.

Constituent: Percentage by weight Molybdenum disulfide (MoS 70 Iron (Fe) 27 Platinum (Pt) 3 Example 12 The following material, typical of composition 2, was formulated by hot pressing technique described above.

Constituent: Percentage by weight Molybdenum disulfide (M08 70 Iron (Fe) 25 Palladium (Pd) 5 Example 13 The following material, typical of composition 3, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Molybdenum disulfide (MoS Iron (Fe) 10 Molybdenum (Mo) 5 Example 14 The following material, typical of composition 4, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Titanium dioxide (T10 50 Nickel oxide (NiO) 15 Nickel (Ni) 35 Example 15 The following material, typical of composition 5, was formulated by the hot pressing technique described above.

Example 16 The following material, typical of composition 2, was formulated by the hot pressing technique described above.

Constituent: Percentage by weight Molybdenum disulfide (MoS 75 Iron (Fe) 20 Palladium (Pd) 5 FIGURE 1 represents a ball bearing assembly for transmission of energy wherein our lubricating compositions are used to advantage In FIGURE 1, number 1 represents an outer race which serves to cover the ball bearing assembly and retain the functioning members therein. A jacket retainer is represented by 2 with each side having a circular jacket retainer with a multiplicity of holes therein represented by 3. The holes in the jacket retainer are adapted to pins 4 so that the two jacket retainers can be held a certain distance apart by the pins by use of the extension 4 on the pins 4. A shaped, lubricant composite material selected from the above compositional ranges is represented by 5 with a hole 6 therein. The holes 6 are adapted to the lubricant composite material 5 so that the pins 4 will fit in holes 6. Each end 7 of the lubricant composite material 5 is shaped so as to retain a ball bearing 8, when the two ends 7 are in close proximity. An inner race 9 serves the function of support for the other parts of the ball bearing assembly previously mentioned as well as an operational base for rotation thereon of the ball bearings 8.

The remarkable lubrication propertiesof our compositions can be demonstrated from tests conducted on the bearing shown in FIGURE 1. The bearing shown in FIGURE 1 is constructed with the lubricant sections selected from the compositional ranges presented above. The bearing is driven by an attached prime mover. The ball bearings in FIGURE 1 as represented by the number 8 were fabricated of stainless steel. After the lubricant composite material was fabricated, X-ray photographs were taken of the separator to determine if internal flaws or cracks existed in the material. The inner race, the outer race, the pins and the jacket retainer were fabricated of standard steel alloy materials.

The assembled bearing, containing the lubricant composite material as fabricated in this invention, was tested in a speed spectrum test at speeds up to 10,000 revolutions per minute (r.p.m.). Operation was for one hour at 1800 r.p.m., one hour at 3600 r.p.m. and one hour at 5000 r.p.m. followed by a life test at 10,000 r.p.m. After four hours of operation at 10,000 r.p.m. the test was terminated due to a general rise in the temperature of the testing equipment and the bearing asembly. The ball bearings and the races were in excellent condition which can be described as exhibiting a high polish and no indication of wear, abrasion, etc. After examination the same bearing with the same components was reassembled for testing. Testing was resumed at 10,000 r.p.m. for 22 hours of operation. At the end of this period the ball bearings and raceways were examined, and these parts again exhibited a high polish with no indications of wear, abrasion, etc.

The tests conducted in air at elevated temperatures and in vacuum at elevated temperatures were run at speeds up to 15,000 r.p.m. for /2-hour periods. No accurate friction wear measurements could be obtained. Of significance is the fact that all bearing assemblies tested for these /2-hour periods were functioning adequately with no sign of failures. Measurements to approximate the material wear for these /2-hour periods varied between .0001 to .0005 inch per exposed surface for the various lubricants. The low wear value resulting from use of our lubricant compositions indicates the excellent wear characteristics for elevated temperatures and the excellent dry film lubricant attained therein.

We claim:

1. A self-lubricating composition of matter for use in air or vacuum comprised substantially as follows:

Platinum (Pt) 2. A self-lubricating composition of matter as claimed in claim 1 wherein the weight percentage of molybdenum disulfide is 90%, iron is 8%, and the platinum is 2%.

3. A self-lubricating composition of matter for use in air of vacuum comprised substantially as follows:

Component- Percentage by weight Molybdenum disulfide (M08 70 to 90 Iron (Fe) 10 to 25 Palladium (Pd) 2 to 5 4. A self-lubricating composition of matter as claimed in claim 3 wherein the weight percentage of molybdenum disulfide is iron is 16%, and palladium is 4%.

5. A self-lubricating composition of matter as claimed in claim 3 wherein the weight percentage of molybdenum disulfide is 75%, iron is 20% and palladium is 5%.

6. A self-lubricating composition of matter for use in air or vacuum comprised substantially as follows:

Component-- Percentage by weight Molybdenum disulfide (M052) 70 to Iron (Fe) 10 to 20 Molybdenum (Mo) 5 to 10 7. A self-lubricating composition of matter as claimed in claim 6 wherein the weight percentage of molybdenum disulfide is 75%, iron is 16.25% and the molybdenum is 8.75%.

8. A self-lubricating composition of matter for use in air or vacuum comprised substantially as follows:

Component- Percentage by weight Titanium dioxide (TiO 45 to 50 Nickel oxide (NiO) 5 to 20 Nickel (Ni) 35 to 45 9. A self-lubricating composition of matter as claimed in claim 8 wherein the weight percentage of titanium dioxide is 45%, nickel oxide is 15% and nickel is 40%.

10. A self-lubricating composition of matter for use in air or vacuum comprised substantially as follows:

Component- Percentage by weight Titanium dioxide (TiO 45 to 50 Nickel oxide (NiO) 5 to 20 Molybdenum disulfide (M05 5 to 20 Nickel (Ni) 15 to 45 11. -A self-lubricating composition of matter as claimed in claim 10 wherein the weight percentage of titanium dioxide is 48%, nickel oxide is 6%, molybdenum disulfide is 6% and nickel is 40%.

12. In a ball bearing assembly, the combination comprising an inner race member supporting a plurality of ball bearings, said plurality of ball bearings being separated by inserted sections of a lubricant composite material comprised substantially as follows:

Constituent- Percentage by weight Molybdenum disulfide (MoS 70 to 90 Iron (Fe) 5 to 27 Platinum (Pt) 1 to 3 said sections of a lubricant composite material being held in place around said inner race by a jacket retainer assembly and said jacket retainer assembly having an outer race adapted to fit around said jacket retainer assembly.

13. In a ball bearing assembly, the combination comprising an inner race member supporting a plurality of ball bearings, said plurality of ball bearings being separated by inserted sections of a lubricant composite material comprised substantially as follows:

Constituent Percentage by weight Molybdenum disulfide (M0S 70 to 90 Iron (Fe) 10 to 25 Palladium (Pd) 2 to 5 said sections of .a lubricant composite material being held in place around said inner race by a jacket retainer assembly and said jacket retainer assembly having an outer race adapted to fit around said jacket retainer assembly.

14. In a ball bearing assembly, the combination comprising an inner race member supporting a plurality of ball bearings, said plurality of ball bearings being sepa rated by inserted sections of a lubricant composite material comprised substantially as follows:

Constituent- Percentage by Weight Molybdenum disulfide (M08 70 to 90 Iron (Fe) 10 to 20 Molybdenum (Mo) 5 to said sections of a lubricant composite material being held in place around said inner race by a jacket retainer assembly and said jacket retainer assembly having an outer race adapted to fit around said jacket retainer assembly.

15. In a ball bearing assembly, the combination comprising an inner race member supporting a plurality of ball bearings, said plurality of ball bearings being separated by inserted sections of a lubricant composite material comprised substantially as follows:

Constituent Percentage by weight Titanium dioxide "(1302) 45 to 50 Nickel oxide (NiO) 5to Nickel -(Ni) to said sections of a lubricant composite material being held Consti-tuent- Percentage by weight Titanium dioxide (TiOg) 45 to Nickel oxide (NiO) 5 to 20 Molybdenum disulfide (M08 5 to 20 Nickel (Ni) '15 to 45 said sections of a lubricant composite material being held in place around said inner race by a jacket retainer assembly and said jacket retainer assembly having an outer race adapted to fit around said jacket retainer assembly.

References Cited by the Examiner FOREIGN PATENTS 2/ 1954 Great Britain.

DON A. WAITE, Primary Examiner.

FRANK SUSKO, Examiner. 

12. IN A BALL BEARING ASSEMBLY, THE COMBINATION COMPRISING AN INNER RACE MEMBER SUPPORTING A PLURALITY OF BALL BEARINGS, SAID PLURALITY OF BALL BEARINGS BEING SEPARATED BY INSERTED SECTIONS OF A LUBRICANT COMPOSITE MATERIAL COMPRISED SUBSTANTIALLY AS FOLLOWS: 